Field of the Invention
The present invention relates, in general to a device for separating, delivering, and depositing stacked blanks of material and more particularly to improvement on West German publication No. 3212741.
According to the device disclosed in West German 32 17 741 if its gripping surface is pressed against an object and the gripping surface with the film of condensed water formed on it during cooling is cooled to below the freezing point by evaporating off a pressurized refrigerant admitted through an inlet opening in an expansion chamber adjacent to the gripping surface, a firm connection is established between the surface of the object and the gripping surface via the frozen condensed water. The object can be lifted and transported to, for example, a processing machine. The firm connection can be dissolved by heating in order to deposit the object. With this device, warm air is blown into the expansion chamber for heating through the same inlet opening through which the refrigerant was fed to the expansion chamber. Special precautions are therefore needed when the refrigerant is to be circulated in a closed cycle. As an alternative, it is stated in the West German publication that the object may be detached from the gripping surface by heating with an electrical heating coil provided in the gripping plate. However, this method requires resistant electrical insulation of the heating coil from the gripping plate, which will impair the heat transfer to the gripping plate.
West German publication 2404863 discloses a design variant of a freezing gripper, in which heating by an electrical resistor is also used instead of warmed air blown-in to thaw the layer of ice in order to dissolve the connection and deposit the object. To achieve this, the gripping plate, which consists of a material with good thermal conductivity, such as copper, and which material is permanently cooled by a cooling medium to a temperature below the freezing point, is provided on its lower side with webs separated by grooves from one another and is covered with an electrically insulating, heat- and cold-resistant plastic film, which film is pressed into the grooves by bars of a holding rake, so that the foil and the bars stand back behind the lower side of the webs in the zone of the grooves. Electrical strip resistors are laid along the lower side of the webs. They are arranged such that they are isolated from the gripping surface and other parts of the device. The spaces between the bottom of the grooves and the foil are filled with air or another heat insulator in order to keep the heat transfer in the zone of the grooves low and to cool to low temperatures only the webs and--through the film--the strip resistors via the gripping plate.
In this arrangement, the objects to be gripped are frozen directly to the exposed side of the strip resistors and are dropped off by heating the strip resistors. What is disadvantageous here is not only that the strip resistors are freely exposed and unprotected, thus representing a source of hazard for the operating personnel, but also the fact that the strip resistors are loaded by the weight of the object to be lifted, as a result of which they are subject to mechanical wear. Diffusion of heat to the crown of the web, which retards the cooling process, can also be expected to occur with this arrangement. Furthermore, the film, being an insulating layer, also hinders the heat transfer from the gripping plate to the strip resistors during cooling.